1. Field of the Invention
The present invention relates to an image forming apparatus and method that is suitably usable for a copying machine, a printer, a plotter, a facsimile machine, or the like.
2. Description of Related Art
One image forming apparatus that has been conventionally known is disclosed in U.S. Pat. No. 3,689,935 in which an electrode having plural openings (hereinafter referred to as "apertures") is used, and a voltage is applied to the electrode in accordance with image data to control passage of toner particles through the apertures, whereby an image is formed on a supporter (image receiving medium) with toner particles that have past through the apertures.
The image forming apparatus includes an aperture electrode unit comprising an insulating flat plate, a reference electrode-formed continuously on one surface of the flat plate, plural control electrodes that are formed on the other surface of the flat plate and that are electrically insulated from one another, and at least one row of apertures each of which is formed in correspondence with each control electrode so as to penetrate through the flat plate, the reference electrode and the control electrodes. The image forming apparatus also includes structure for selectively applying a voltage across the reference electrode and the control electrodes, structure for supplying charged toner particles so that the flow of the toner particles passed through the apertures is modulated in accordance with the applied voltage, and structure for moving a supporter and the aperture electrode unit relative to each other to position the supporter in a particle flow passage.
U.S. Pat. Nos. 4,743,926, 4,755,837, 4,780,733, and 4,814,796 disclose image forming devices having an aperture electrode unit disposed so that control electrodes face a supporter and a reference electrode faces a toner supply side.
On the other hand, U.S. Pat. No. 4,912,489 discloses an aperture electrode unit disposed so that the reference electrode faces the supporter, and the control electrodes face the toner supply side. The reference describes that a voltage to be applied to the control electrodes at an off-time can be reduced to about a quarter of that of the image forming apparatus as disclosed in the above-mentioned patents.
The term "off-time" means a time when no toner particle is attached onto the supporter, that is, when a blank portion of an image is formed on the supporter, and conversely, the term "on-time" means a time when a toner image is formed on the supporter.
However, in the conventional image forming apparatus as described above, the voltage difference between the control voltages at the on-time and at the off-time (which is referred to as "driving voltage") is driven by an integrated circuit, and thus, it is preferably set to a small value. Practically, it must be set to 50 V or less. The relationship between the control voltage to be applied to the control electrodes and the density of an image formed on a supporter (image receiving medium) is shown as indicated by a dotted line in FIG. 4A. Even when the control voltages at the on-time and at the off-time are set to any values while maintaining the driving voltage at 50 V, it has been impossible to perform such a printing operation with the image density above 1.5 at the on-time and below 0.07 at the off-time without fog.
For various reasons, in the relationship between the control voltage and the image density as shown in FIG. 4A, when a plus voltage is applied to the control electrodes at the on-time to generate toner flow, the toner flow is generated in accordance with the control voltage. Conversely, when a minus voltage is applied to the control electrodes at the off-time, no toner flow is generated in accordance with the control voltage.
For example, as indicated by the dotted line of FIG. 4A, when the control voltage at the on-time is set to 70 V and the control voltage at the off-time is set to 20 V, the image density above 1.5 is obtained at the on-time, which is a sufficient value. At the off-time, image fog occurs, and thus, the image density is not sufficiently reduced, and image quality is deteriorated. On the other hand, when the control voltages at the on-time and at the off-time are set to 30 V and -20 V, respectively in order to suppress the image fog, the image fog is suppressed at the off-time, and the image density is sufficiently reduced to about 0.07, which corresponds to the density of a sheet (background). However, the image density at the on-time becomes insufficient, and thus, the image quality is deteriorated.
The relationship between the control voltage and the image density should be stepwisely (binary) varied at a threshold voltage as indicated by a solid line of FIG. 4B, and in this case, the control voltage can be reduced to an extremely small value. However, there is a dispersion of various toner characteristics such as the charge amount, particle diameter, etc. of individual toner particles to be supplied to the toner particle control means. Therefore, when the respective toner particles that are dispersed having the toner characteristics as described above are bundled into toner flow as a whole, the actual relationship between the control voltage and the image density is shown by a dotted line in FIG. 4A, and thus, the control voltage is increased.
Further, if the adhering force between the toner and the carrier is made uniform, the density curve shown in FIG. 4A could approach the ideal curve as described above, and it is effective to carry the toner as a single layer on the carrier. However, in this case, the toner directly suffers an effect of dispersion in the particle diameter, the charge amount, the weight, etc. of the toner because it is a single layer, and thus, a more uniform characteristic is required for the toner to efficiently approach the ideal density curve. In a case of toner that is manufactured with a grinding method or the like and used for an electrophotographic apparatus or the like, the degree of dispersion of the toner is set to about 1.3 to 1.4. The degree of dispersion is defined as a value that is obtained by dividing an average particle diameter obtained from a particle-diameter distribution on the basis of the toner particle volume by an average particle diameter obtained from a particle-diameter distribution on the basis of the toner particle number.
For example, in FIG. 7, a distribution X at the left side of FIG. 7 represents a particle-diameter distribution on the basis of the number of particles, and a distribution Y at the right side of FIG. 7 represents a particle-diameter distribution on the basis of the volume of the particles. The average particle diameter obtained from the particle-diameter distribution X is equal to 7 .mu.m, and the average particle diameter obtained from the particle-diameter distribution Y is equal to 9.5 .mu.m. That is, these average particle diameters are inconsistent with each other, and thus, the toner characteristic is not uniform carrying the toner as a single layer.